Optical transmission systems using optical amplifiers and wavelength division multiplexing

ABSTRACT

The invention provides a method and system for transmitting very high bit rates over an optical link, using unidirectional and bidirectional WDM technology. The 1550 nm window for optical transmission is spatially separated into two bands, &#34;Red&#34; and &#34;Blue&#34;, and the channels in each band are selected so that respective wavelengths present a substantially equal gain tilt. A fiber amplifier designed according to spatial separation and wavelength selection of the invention is disclosed, along with multiple span WDM network topologies.

This application is a divisional of application Ser. No. 08/669,929, nowU.S. Pat. No. 5,801,858 filed Jun. 25, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to optical transmission systems and methodsfor transmission of information over optical networks, and moreparticularly to optical transmission systems and methods that useoptical amplifiers and wavelength division multiplexing.

2. Background Art

The performance of standard reach optics limits the spacing between theline terminating equipment (LTE) and regenerating equipment toapproximately 80 km (20 dB at 0,25 dB/km) on non-dispersion shifted ordispersion shifted optical fiber. The 80 km limitation is caused byphysical degradation of the transmitted optical signal due to opticaldispersion and optical attenuation. The dispersion and attenuationlimits, respectively, can both be extended to beyond 80 km with theintroduction of external modulation, use of dispersion shifted opticalfiber, optical amplifier technology and wavelength division multiplexing(WDM) technology.

When designing multiband transmission systems, three significant issuesto consider are the number of amplifiers required (1), gain tilt (2),and protection (3).

(1) Optical amplifiers are expensive units and so the number and typesof units required to implement a given data connection is an importantdesign parameter for an optical network.

There are three general types of optical amplifiers: post-amplifiersthat connect to a transmitter to boost he output power; line amplifiersthat amplify along the route; and preamplifiers that improve thesensitivity of optical receivers. These different types of amplifiersprovide different output powers, use different input power levels, andgenerally have different noise figure requirements.

One way of reducing the number of optical amplifiers is to use thewavelength division multiplexing (WDM) technology. Use of the WDMtechnology reduces the strands of optical fiber cable needed toestablish a communication link, and it can provide manifold capacityexpansion on existing fiber links. Its potential for routing signals isequally important.

For example, the WDM filters perform the function of coupling the pumpsource laser wavelength to the erbium doped fiber. Three-port WDMfilters are currently used to couple multiple wavelengths into and outof the transmission fiber.

A four-port WDM coupler for implementing a bidirectional opticalamplifier module using a unidirectional optical amplifier is disclosedin U.S. Pat. No. 5,452,124 (Baker, issued Sep. 19, 1995 and assigned toWilliams Telecommunications Group).

Isolators are also equipment used in WDM systems, and they function toallow an optical signal to pass in a single direction. If opticalisolators are used internal to an optical amplifier, then they make theamplifier an inherently unidirectional device. It is known to useisolators inside gain regions of an optical amplifier. U.S. Pat. No.5,280,549 (Barnard et al, issued on Jan. 18,1994 and assigned toNational Research Council of Canada) discloses a frequency dependentoptical isolator which allows signals to pass in one direction only, sothat two signals may be isolated according to their frequencies.

(2) The use of erbium doped fiber amplifiers (EDFA) for multichannel,bidirectional transmission is current practice. Of great importance innetwork applications is the configuration of the optical amplifier andwhat signal wavelength to use in conjunction with the pump wavelength.Because the amplifier gain is not perfectly flat for all incomingwavelengths, the precise wavelengths to use is a function of the gainvariations of the different available pumps. Gain tilt is a significantissue to consider when designing multiband transmission systems. Gaintilt measures the change in the profile of the gain for eachtransmission channel at the actual value of the gain of the amplifiermodule, with respect to the gain profile at the nominal value of thegain, i.e. at the value for which the amplifier is designed. In otherwords, the gain tilt function varies with link loss. This functiondepends only on the physics of the dopant in the host fiber glass, andis of interest when signals of more than one channel or direction sharethe same fiber.

No chemical solutions have yet been found for addressing the gain tiltproblem. Dopants, fluoride, etc. can help flatten the gain profile, butdo not solve the tilt. Electronic solutions are currently underinvestigation.

One solution is "gain clamping", which means maintaining the amplifiergain constant on all channels with an idler or lasing. However, thissolution requires use of twice the number of laser pumps to provide thenecessary extra photons.

Another solution is "loss padding", which implies tuning the loss ofeach span to match the nominal value for the amplifier or, in otherwords, to operate all amplifiers of the link at their nominal gains.This solution has the disadvantage of requiring attenuators to be placedin each span, and is not very robust in the presence of variations inlosses and optical powers in the system over time and temperature. "Gainclamping" methods combined with "loss padding" slightly improve thesystem's robustness, at the price of much more expensive pump lasers.

Another solution to address the gain tilt problem is to use anadjustable optical filter. The relative loss between differentwavelengths could then be adjusted by a mechanical or electricalcontrol. The best location for such a filter is inside the amplifier.The filter requires adjustability in the field or, better yet, to becontinuously controlled by measuring each wavelength power level. Thesefilters may become more affordable in a few years, but they arecurrently very expensive and therefore not used.

The prior art fails to provide cost effective solutions foramplification of bidirectional multi-channel optical signals. Inaddition, effective implementation of four-port WDM filters is difficultbecause of the loss introduced by the filter, gain tilt and protectionproblems. The prior solutions and configurations are not concerned withcontrol of the gain tilt or with protection of transmission inmulti-channel amplifiers.

(3) To ensure the desired availability of network connections ismaintained, it is standard practice in the telecommunication industry toimplement redundant equipment so that should one unit fail, another canbe rapidly switched into place. This is called protection switching. Thenumber and the type of amplifiers that need to be held as replacementunits is also important. Reducing the number of different types ofequipment in a network reduces the number of types of spare units and,through standard sparing statistics, significantly reduces the totalnumber of spare units that the network operator must purchase and havereadily at hand.

Four general types of protection switching are known: "1+1" protection,whereby one set of equipment protects another set of equipment on amatched pair basis; "1:N" protection, whereby one set of equipmentprotects N other sets; "ring" protection; and "cross-connect"protection.

Protection protocols can be configured as "bidirectional switching" and"unidirectional switching". The protection protocol has nothing to dowith the direction of transmission on the fiber; it is just theswitching protocol type. Telecommunication traffic may be bidirectionalin nature, as for example, voice circuits, or unidirectional, as forexample, CATV signals. Bidirectional traffic means that the data istransmitted in both directions. Bidirectional also means that while agiven circuit is interrupted in one direction, there is minimal penaltyto interrupt the other direction of the same circuit.

A unidirectional protection switch switches only one direction of acircuit, namely, that direction requiring protection if only onedirection is degraded. In contrast, a bidirectional protection switchtries to switch both directions together in all cases.

A ring topology with "1+1" protection offers significant advantages incomparison with a linear, or "1:N" topology. When more than onewavelength is carried by one optical amplifier and only one signal at atime can be protected, such as in a "1:N" system, then when thatamplifier fails some of the signals will not be protected. This severelyimpairs the availability of circuits carried within those signals. Themethods below allow signals with multiple wavelengths through oneamplifier to be efficiently protected.

Electrical, and soon optical, cross-connects can implement the above andmore general protection or restoration topologies. However,cross-connects are not generally as fast to protect as transmissionequipment implementing the above three protection methods, and aretherefore generally used to implement restoration rather thanprotection.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide multiple wavelengthtransmission systems with optical amplifiers and wavelength divisionmultiplexing, which reduce or alleviate some or all of the drawbacksencountered with the prior art solutions.

It is another object of this invention to provide an improved approachto the design of multi-channel transmission systems for allowingmultiple wavelengths to be amplified by the same optical amplifier andtransmitted over the same optical fiber.

Still another object of this invention is to provide multiple wavelengthsystems with a reduced number of amplifiers and other optical componentssuch as dispersion compensation modules, which allows the system totolerate more loss from the outside plant fiber cable.

Yet another object of the invention is to provide multiple wavelengthsystems with a reduced number of optical components which implicitlyrequire a reduced number of spare components for protection facilities.

Accordingly, the invention is directed to a wavelength divisionmultiplexing (WDM) optical amplifier module comprising a firstperipheral gain region for amplifying a first multichannel opticalsignal; first splitter means connected to the first peripheral gainregion for spatially separating the first multi-channel optical signalinto a first band optical signal and a second band optical signalaccording to the wavelength; a central gain region for receiving thefirst band optical signal and the second band optical signal, separatelyamplifying same, and providing an amplified first band optical signaland an amplified second band optical signal; second splitter means forrecombining the amplified first band optical signal with the amplifiedsecond band optical signal into a second multi-channel optical signal;and a second peripheral gain region connected to the second splittermeans for amplifying the second multi-channel optical signal.

According to another aspect, the invention comprises an opticalbidirectional regenerator for multi-channel telecommunication usingwavelength division multiplexing, comprising first splitter means forspatially separating a first multi-channel optical signal into a firstband optical signal and a second band optical signal according to thewavelength; a bidirectional regenerator module for receiving the firstband optical signal and the second band optical signal from the firstsplitter means, separately amplifying same, and providing an amplifiedfirst band optical signal, and an amplified second band optical signal;and second splitter means for recombining the amplified first bandoptical signal with the amplified second band optical signal into asecond multi-channel optical signal.

The invention also provides a method for transmitting multi-channeloptical signals between a first and a second site connected by anoptical transmission path including a fiber amplifier, comprising thesteps of spatially separating the gain region of the fiber amplifierinto a first gain region and a second gain region; associating the firstgain region with a first transmission band and the second gain regionwith a second transmission band; transporting a first optical signal inthe first transmission band; and transporting a second optical signal inthe second transmission band.

In addition, the invention provides for a method for transmittingmulti-channel optical signals between a first and a second node of atransmission link comprising the steps of defining a first transmissionband associated with a first direction of transmission and a secondtransmission band associated with the opposite direction oftransmission; transferring, between the first node and a first end of afiber span, a first multi-channel optical signal comprising a firstoptical signal S1_(m), where mε [1,M] and M is an integer, in the firsttransmission band, and a second optical signals S2_(k), where kε [1,K]and K is an integer, in the second transmission band; spatiallyseparating on the fiber span the first optical signal S1_(m) along afirst unidirectional route, and the second optical signal S2_(k) along asecond unidirectional route, in accordance with the wavelength;separately amplifying the first optical signal S1_(m) and the secondoptical signal S2_(k) ; and combining the first optical signal and thesecond optical signal to form a second multi-channel optical signal andtransferring same between the transmission link and the second node.

A major advantage of the optical transmission systems according to thisinvention is that multiple wavelengths may be implemented in the 1550 nmwindow of the band of an Erbium doped fiber amplifier (EFDA) withoutsevere gain tilt impairments. This allows reduction of the number ofdifferent types of equipment in a network, which in turn reduces thenumber of types of spare units, and through standard sparing statistics,significantly reduces the total number of spare units that the networkoperator must purchase and have readily at hand.

For example, four channels per direction may be supported with aconfiguration using spatial separation and WDM wavelength choiceaccording to this invention.

Another major advantage of the optical transmission systems according tothis invention is that reduced fiber counts need to be used fordeploying such systems, which is especially important when there is alack of fibers on a given route. As a result, transmission systemsaccording to this invention also reduces the system cost, when more thantwo wavelengths are amplified per amplifier. The reduced impact of theloss of the other optical components allows the system to tolerate moreloss from the outside plant fiber cable.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments, as illustrated in the appendeddrawings, where:

FIG. 1A shows a configuration of a transmission system using three-portWDM splitters;

FIG. 1B shows a configuration of a transmission system using a four-portWDM splitter;

FIG. 2A shows the gain for two channels when the amplifier operates atthe nominal gain;

FIG. 2B illustrates the gain for the two channels of FIG. 1A when theamplifier module operates at a gain lower than the nominal gain;

FIG. 2C is a schematic graph showing the gain tilt per dB of gain changefor the bands of interest;

FIG. 3 illustrates a block diagram of an optical amplifier using spatialseparation;

FIG. 4 is a block diagram of a bidirectional regenerator;

FIG. 5 shows three examples of selection of the wavelength formulti-channel optical transmission systems;

FIG. 6 is a multiple span optical transmission system configurationusing common pre/post WDM amplification modules;

FIG. 7A is a paired optical transmission system showing the economy inthe fiber spans for working and protection links of a 1:1 linearconfiguration;

FIG. 7B illustrates paired two-stacked, two fiber rings using WDMamplifier modules; and

FIG. 7C is a paired four-fiber ring configuration using WDM amplifiermodules.

DESCRIPTION OF THE PREFERRED EMBODIMENT

WDM couplers or splitters are filters used to separate or combineoptical signals according to their wavelength. For example, a WDMcoupler is used to couple the light from the laser source into the gainregion of an EDFA. These couplers are very useful in multi-channeloptical systems, however, the isolation requirements in the WDM could bea severe issue due to the multi-path interference from imperfectisolation of the splitters. Three-port conventional WDM couplers may beused in a configuration as illustrated in the example of FIG. 1A. FIG.1A shows a bidirectional connection between sites A and B where channelsof wavelengths μ₁ and λ₂ use the same fiber 1. WDM splitter 2 at site Aconnects transmitter Tx1 to fiber 1 through ports 4 and 3, for launchingoptical signal S1 of wavelength λ₁ on fiber 1. WDM splitter 2 alsoconnects fiber 1 to receiver Rx2 through ports 3 and 5, for directingoptical signal S2 of wavelength λ₂ from fiber 1 to receiver Rx2.Similarly, WDM splitter 6 is connected with port 7 to fiber 1 and withport 8 to transmitter Tx2 for launching optical signal S2 on fiber 1,while port 9 is connected to receiver Rx1 for directing the opticalsignal S1 from fiber 1 to receiver Rx1.

The four-port filter disclosed in the previously identified U.S. Pat.No. 5,452,124 (Baker) may be used for obtaining bidirectionalamplification using a unidirectional amplifier. A bidirectional systemconfiguration using such a four-port WDM splitter is illustrated in FIG.1B. Sites A and B have a similar configuration with that of FIG. 1A.Optical signal S1 of wavelength λ₁ travels from site A to site B, whileoptical signal S2 of wavelength λ₂ travels in an opposite direction ofpropagation, namely from site B to site A. The four-port WDM splitter 10is provided at a site C, together with a unidirectional opticalamplifier 15. Splitter 10 routes the traffic so that both channels havethe same direction in the gain portion of the amplifier 15. Thus,signals S1 and S2 received at ports 11 and 12, respectively, are routedin the same direction to exit at port 13, which is connected to theinput port of optical amplifier 15. The amplified signals S1 and S2arrive at port 14 of the WDM filter 10 in the same direction oftransmission and are launched on fiber 1 in the appropriate direction,according to the wavelength λ₁ or λ₂.

The number of channels and the wavelength of each channel are importantdesign parameters for optical transmission systems. It has been notedthat the gain of an optical amplifier changes when the power at theinput is different (lower or higher) than the nominal power for whichthe amplifier was designed. This change in the gain with the inputpower, defined as "gain tilt", is also dependent on the channelwavelength. As an example, FIG. 2A shows the gain of transmissionchannels 1 and 2 when the amplifier module operates at the nominal gain.FIG. 2B shows the changes in the gain of channels λ₁ and λ₂ when theactual value of the gain is lower than the nominal value. Operating at ahigher gain would produce a profile with the opposite slope.

Today's optical amplifier technology works well for single wavelengthamplification. However, the gain curve across the 1550 nm passband ofinstalled, non-dispersion fiber is not flat, especially in the 1530 to1542 nm range. FIG. 2C shows the change in gain at each wavelength ifthe gain at 1545 nm is increased by 1 dB. For example, the gain at 1532nm would go up 1.8 dB for each 1.0 dB increase at 1545 nm. This is1.8-1.0=0.8 dB of tilt/dB of change in span loss.

WDM is not generally used in the 1530-1542 nm range because of the gainpeak with silica-based EDFAs which causes per-channel output power tovary greatly in this region and seriously affects signal gain outsidethis region when several amplifiers are cascaded. The current solutionsto address this problem include selectively varying the input power foreach channel, or staying in the region of flat gain. The first solutioncauses a significant reduction of the output power of the individualchannels, and makes the span engineering difficult. The second solutionlimits the number of channels that can be multiplexed, and also preventsthe reuse of the large embedded base of OC-48 transmitters, without theaddition of expensive wavelength adapters.

According to this invention, the optical amplifier gain region of about1528 nm to 1560 nm is split into two bands: "Blue" for 1528 to 1543 nm,and "Red" for 1547 to 1560 nm. The two bands are spatially separated foramplification. The wavelength within a band is chosen to be in theregions of similar gain tilt, or to have minimum variation of the gaintilt/dB in the respective "Red" or "Blue" band. This is especiallyimportant for the "Blue" gain region, which has large variations in gaintilt, as shown in FIG. 2C.

FIG. 3 illustrates a block diagram of a bidirectional fiber amplifier,as an example an erbium doped fiber amplifier (EDFA) 15 using spatialseparation according to this invention. In this disclosure, the terms"active optical fiber" and "gain region" designate a length of opticalfiber doped with a rare earth element and pumped with coupled waves foramplifying optical signals.

As shown in FIG. 3, the optical signals traveling within the centralsection of optical amplifier 15 are grouped in two bands, each for adirection of transmission in this example. The traffic in the Red andBlue bands is physically separated on fibers 17 and 19 using a firstthree-port WDM splitter 21. Signals on fibers 17 and 19 are thenrecombined by a second three-port WDM splitter 23. As a result, theamplifier has two peripheral gain spans 25 and 29, and two central gainlengths 27 and 31. The signals travel in both directions at theperiphery of the amplifier in the gain spans 25 and 29 and in onedirection in the central lengths 27 and 31. Isolators 33,35, and 37 arealso provided and oriented so as to obtain bidirectional operationbetween the bands. Because the "Red" and "Blue" gain lengths arespatially separated, separate power control per direction is preferablyused, through separate saturation of gain and through control ofseparate pump powers. Embodiments of the amplifier module 15 where thesignals travel in the same direction may also be designed, in which caseseparate or common control per band may be used.

This architecture significantly reduces the effect of gain tilt betweenthe two bands and multi-path interference from imperfect isolation ofthe two splitters. By using different directions in the central regionof the EDFA, and also two spatially separated bands, the isolationrequirements in the WDM splitters 21 and 23 are achievable. In addition,by separating the bands within the amplifier and providing gain stagesoutside the separation, the noise figure and output power penalty due tothe losses of the WDM couplers are reduced.

The optical fiber exhibits different dispersion characteristicsdepending on the wavelength used for transmission. For standard singlemode fiber, the two main transmission windows of interest are at 1310 nmfor non-dispersion shifted optical fiber, and 1550 nm for dispersionshifted optical fiber. When non-dispersion shifted optical fiber isused, the transmitter should preferably be equipped with dispersioncompensating modules (DCM) for reducing the significant transmissionimpairment at high bit rates.

FIG. 4 is a block diagram of a bidirectional regenerator built inaccordance with the spatial separation method of the invention.Inserting a four-port WDM router 41 into a fiber carrying bidirectionalWDM signals separates the incoming signals according to their directionof propagation. The signals in the "Blue" band arrive, for example, atport 11 on fiber 1', and the signals in the "Red" band arrive at port 12on fiber 1. These signals exit at port 13 and are routed by WDM router41 in the same direction from port 13 to the input of a preamplifier 42,then to a dispersion compensating module 43. A three-port WDM splitter44 separates the signals according to their wavelength and routes themto receivers Rx1 and Rx2 of block 40. After regeneration or add-dropmultiplex functions are performed in block 40, the signals are passed totwo transmitters Tx1 and Tx2. From the two transmitters, the signals arecombined with a three-port WDM splitter 45, passed through anotherdispersion compensation module 46 if needed, and then amplified by apost-amplifier 47. The four-port WDM router 41 receives the amplifiedsignals at port 14 and inserts these signals into the two fibers in theappropriate directions.

FIG. 4 also illustrates how the configuration compensates for the lossintroduced by the WDM couplers and DCMs presently available.

Selection of the wavelengths within the Red and Blue bands is alsoimportant in multi-channel systems for further reducing the gain tilt.FIG. 5 shows three examples of selection of the wavelength within aband, for obtaining up to eight wavelengths evenly split between the twodirections. In the first example, two channels in the "Blue" band andtwo channels in the "Red" band are selected. The wavelength of the first"Blue" channel is λ₁ =1533 nm and that of the second "Blue" channel isλ₂ =1541 nm. The wavelength of the first "Red" channel is λ₃ =1549 nmand that of the second "Red" channel is λ₄ =1557 nm. The difference ingain tilt for the two "Blue" channels is relatively large, and it isshown in FIG. 2C at 0.65, while the difference in gain tilt for the two"Red" channels is 0.1. A gain tilt of 0.65 will cause problems in thesystem.

The second example shows another selection with two channels in the"Blue" band and two channels in the "Red" band. The. wavelength of the"Blue" channels is λ₁ =1528.7 nm and λ₂ =1533.5 nm having approximatelythe same gain tilt of 1.75. The wavelength of the "Red" channels is λ₃=1552.5 nm and λ₄ =1557.4 nm having a difference in gain tilt ofapproximately 0.1, and thus very little difference. This small gain tiltwill not cause problems in the system of this example, because thewavelengths of the channels were selected to have a matched gain tilt.

The third example shows a system with eight channels, with four channelsin the "Blue" band and four channels in the "Red" band. The wavelengthof the "Blue" channels is λ₁ =1528.7 nm, λ₂ =1530.3 nm, λ₃ =1531.9 nmand λ₄ =1533.5 nm, having a variation in the gain tilt of a maximum of0.1. The wavelength of the "Red" channels is λ₅ =1550.1 nm, λ₆ =1552.5nm, λ₇ =1554.9 nm and λ₈ =1557.4 nm, with a difference in gain tilt ofapproximately 0.1.

This method of selection of the wavelength of a transmission channelallows multiple wavelengths to be amplified in the Blue band of anerbium doped fiber amplifier without severe gain tilt impairments.

In addition, active equalization between wavelengths may also beobtained by adjusting the transmitted power and also by providingin-line equalization devices when they become available.

If multiple channels are transmitted on a single fiber span andamplified in the same module or modules, the number of amplifiers, fiberspans, and other optical components between two terminals can beminimized. There are several ways of achieving this reduction in fiberspans and amplifiers based on spatial separation of the transmissionbands according to the direction of propagation, and on the properselection of signal wavelength. Some examples will be presented in thefollowing.

A receiver typically requires -5 dBm of optical power and overloads at 0dBm, and a transmitter is typically required to launch into the outsideplant fiber +7 or +10 dBm. These design parameters make it difficult orimpossible to design an optical amplifier which simultaneously operatesas a pre- and post-amplifier. In addition, these requirements areaggravated by the presence of gain tilt. To obtain these power levels,the current transmitters are provided with an optical amplifierconfigured as a post-amplifier and the receivers are provided with anoptical amplifier configured as a pre-amplifier.

However, a single fiber amplifier may operate simultaneously as a pre-and post-amplifier according to this invention, by providing separatepower control per direction and selecting the wavelength of the channelsfor minimizing the gain tilt.

FIG. 6 shows a configuration with bidirectional amplifiers operating asline amplifiers and as combined pre- and post-amplifiers. In thisconfiguration, there are four signals, λ₁, λ₂, λ₃ and λ₄, in the "Red"band, and one signal, λ₅, in the "Blue" band. The DCM 43 and the WDMsplitter 2 at site A are separated from the optical cable 62 by the gainof the pre/post-amplifier 60. Similarly, the DCM 45 and the WDM splitter6 at site B are separated from the optical cable span 63 by the gain ofthe pre/post-amplifier 61. This reduces the impact of the optical lossof these components on noise and on power level.

Optical amplifiers 60 and 61 are provided with separate power controlper direction, so that amplifier 60 operates as post-amplifier fortransmitters Tx1-Tx4 and as pre-amplifier for receiver Rx. Similarly,amplifier 61 operates as a pre-amplifier for receivers Rx1 to Rx4 and asa post-amplifier for transmitter Tx.

Appropriate selection of wavelength combined with the spatial separationof the channels according to this invention, allows for furthersimplification of the configuration of a multi-channel system. Thus, ifthe number of transmission channels in one direction equals the numberof channels in the opposite direction, a cost effective network may beobtained by reducing the number of protection links.

FIGS. 7A, 7B and 7C illustrate systems with an equal number of signalsin both bands, here called bidirectional pairing. Thus, for eachtransmitter-receiver path in one direction, there is areceiver-transmitter path in the opposite direction. This configurationhas significant advantages and reduces the number of protection links.With an appropriate flattening filter, a WDM amplifier module as shownin FIG. 3 can be designed to support an equal number of wavelengths inboth directions. It is recommended that working and protection groups beprovisioned through separate amplifiers in order to prevent exposure toa single amplifier failure.

FIG. 7A is a balanced bidirectional optical transmission system showingthe economy in the fiber spans for working and protection links of alinear configuration of a single 1:1 system. One set of amplifiers 52and 53 is equipped to accommodate the working traffic on fiber 1. Theprotection traffic takes place on a separate route 20 using amplifiers54, 55, 56, and 57. No more amplifiers are necessary to support a second1:1 system since the amplifiers support two "Red" band and two "Blue"band wavelengths. Further systems may be provisioned to use the samefiber and amplifiers, provided that appropriate wavelengths areavailable and the link budget can be met. Compared to a system that doesnot use this pairing, such as a unidirectional WDM system, a pairedbidirectional system requires fewer channels to carry and protect agiven amount of data. Fewer channels means fewer fibers, feweramplifiers, and fewer terminal electronics required to carry the samedata.

FIG. 7B is a configuration of two-stacked, two-fiber rings using WDMamplifier modules as shown in FIG. 3. Because of the bidirectionalnature of the amplifiers, only one fiber per span is required toimplement a base system. No more amplifiers are thereafter required toimplement a second system. Further ring systems can be provisioned touse the same fiber and amplifiers, if appropriate spatial separation andpaired wavelength selection are used, as discussed in connection withFIGS. 2C and 5, and the link budget can be met.

FIG. 7C is a four-fiber ring configuration using the WDM amplifiermodules. A four-fiber ring may be converted to a "two-fiber ring withspan switching". This differs from a normal two-fiber ring in that thefour-fiber ring protocol is implemented, allowing such features as spanswitching.

While the invention has been described with reference to particularexample embodiments, further modifications and improvements which willoccur to those skilled in the art may be made within the purview of theappended claims, without departing from the scope of the invention inits broader aspect

What is claimed is:
 1. A method for transmitting multi-channel opticalsignals between a first site and a second site connected over an opticaltransmission path including an optical amplifier, comprising the stepsof:defining a first transmission band and a second transmission bandwithin a rain region of said optical amplifier; measuring a variation ofthe gain of said optical amplifier with input power and wavelength;selecting at least two first band wavelengths in said first transmissionband, and at least one second band wavelength in said secondtransmission band, characterized in that the gains for said first bandwavelengths are substantially similar to each other, irrespective of theinput power; and associating said first band wavelengths with adirection of transmission and said at least one second wavelength withan opposite direction of transmission.
 2. A method as claimed in claim1, wherein said first transmission band and said second transmissionband are spatially separated within said gain region of said opticalamplifier.
 3. A method as claimed in claim 1, wherein said firsttransmission band is defined in a blue region of a spectrum, and saidsecond transmission band is defined in a red region of the spectrum. 4.A method as claimed in claim 1, wherein a number of said first bandwavelengths is equal to a number of said second band wavelengths.
 5. Amethod as claimed in claim 1, wherein said step of selectingcomprises:preparing a gain tilt/dB graph for said first transmissionband; and selecting on said graph said first band wavelengths to havesubstantially similar gain tilt values on an abscissa of said graph. 6.A method as claimed in claim 5, wherein a difference in gain tiltbetween any two of said first band wavelengths is less than 0.65.
 7. Anoptical amplifier of a multi-channel optical transmission systemcomprising:means for spatially separating a gain region of said opticalamplifier into a forward path and a reverse path, each of said forwardpath and said reverse path being associated with a continuoustransmission band; and means for directing at least two first bandsignals over said forward path and at least one second band signal oversaid reverse path according to a wavelength of said respective signals,wherein the wavelengths of said at least two first band signals areselected to have substantially similar gain tilt values.